Research Interests

I use N-body simulations to compute the evolution of isolated
galaxies. I have made my simulation code public through the website
GALAXY, with a
detailed on-line manual at
this url

I am also the author, with Kristine Spekkens, of the public
software package DiskFit that can be used to fit disk galaxy
models to either photometric images or kinematic data. DiskFit
can be downloaded
from this url.

Posted here is an electronic version,
with full quality figures, of "Dynamics of Barred Galaxies" by myself
and A. Wilkinson that appeared in Reports on Progress in
Physics (1993) 56, 173

My main interests are structure and evolution of galaxies, their
formation and their dark matter content. The following are some of
the topics I have been working on recently:

Dark Matter The extended flat rotation curves of spiral
galaxies provide some of the strongest evidence for mass discrepancies
in galaxies, and are usually interpreted as evidence for dark matter
halos. My
research
suggests that dark matter halos have large low density cores,
i.e. most of the dark matter lies in the outskirts of galaxies
and that the inner parts of galaxies are dominated by the mass in the
stars.

Spiral Structure Most disc galaxies display graceful
spiral patterns, yet over a century and a half, after their first
discovery, we still do not fully understand how they originate! My
view
is that the spirals are short-lived, recurring structures caused by
real instabilities (see also). I have
obtained some observational
support for this point of view, using data from
ESA's Hipparcos satellite.

Bar Stability About 30% of galaxies are strongly barred
and a roughly equal fraction are weakly barred, while the remaining
significant minority do not have bars. We do not yet understand the
reasons for these proportions. It has been suggested that galaxies
with less dark matter develop bars through a global instability, but
there is no evidence to support this speculation. My
work
suggests that whether a galaxy is barred or not today may depend on
the distribution of angular momentum in the material from which it
formed.

Formation of massive Black Holes and QSO activity I
argue
that a bar must develop early in the life of nearly every galaxy and
that gas to create and fuel the QSO is driven into the center of the
galaxy by the bar. The QSO lifetime, and the mass of its central
engine, are also controlled by large-scale dynamics, since the fuel
supply is shut off after a short period by the development of an inner
Lindblad resonance. This resonance causes the gas inflow along the bar
to stall at a distance of a few hundred parsecs from the center. The
ILR develops as a result of previous inflow, making quasar activity
self-limiting. The bars are weakened and can be destroyed by the
central mass concentration formed in this way.

Warps Many disk galaxies are not perfectly flat, but the
outer parts are bent away from the plane defined by the inner disk,
especially in the gas. Theoretical work suggests that this state of
affairs could not persist for the believed ages of galaxies and that
some mechanism is required to excite the warp continually (or
repeatedly). With my student, Juntai Shen, we
show
that realistic warps can be excited by late infalling matter having a
misaligned angular momentum vector. Our simulations manifested
realistic warps, and a series of simplified experiments allowed us to
understand the complex dynamical interaction of the disk and halo with
the externally applied torque. Furthermore, we were able to show that
the persistence of warps is not as puzzling as is widely believed;
halo damping of the warp is very weak since the inner halo remains
aligned with the slowly precessing disk and is only weakly coupled to
the outer halo. While these idealized experiments have not settled
all outstanding questions, they have shown that the problem may be
closer to a solution than is widely thought.

Numerical Methods for N-body simulations I have
compared
five different methods for simulating galaxy models, which shows
that grid based (or particle-mesh) methods are superior in efficiency
to any other and are quite versatile. The combination of efficient
codes and quiet starts allows many galactic dynamical problems to be
tackled without the need for supercomputers.
See my lectures